求大神讲解36套中的EXERCISE7中的一题

瓶子要学习UID: 2999456

Flatfish, such as the flounder, are among the few vertebrates that lack approximate bilateral symmetry (symmetry in which structures to the left and right of thebody‘s midline are mirror images). Most striking among the many asymmetries evident in an adult flatfish is eye placement: before maturity one eye migrates, so that in an adult flatfish both eyes are on the same side of the head. While in most species with asymmetries virtually all adults share the same asymmetry, members of thestarry flounder species can be either left-eyed (both eyes on the left side of head) or right-eyed. In thewaters between the United States and Japan, the starry flounder populations vary from about 50 percent left-eyed off the United States West Coast, through about 70 percent left-eyed halfway between the United States and Japan, to nearly 100 percent left-eyed off the Japanese coast.
Biologists call this kind of gradual variation over a certain geographic rang a ―cline‖ and interpret clines as strong indications that the variation is adaptive, a response to environmental differences. For the starry flounder this interpretation implies that a geometric difference (between fish that are mirror images of one another) is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a perplexing questions: what is the selective advantage in having both eyes on one side rather than on the other?
The ease with which a fish can reverse the effect of the sidedness of its eye asymmetry simply by turning around has caused biologists to study internal anatomy,especially the optic nerves, for the answer. In all flatfish the optic nerves cross, so that the right optic nerve is joined to the brain‘s left side and vice versa. This crossing introduces an asymmetry, as one optic nerve must cross above or below the other. G. H. Parker reasoned that if, for example, a flatfish‘s left eye migrated when the right optic nerve was on top, there would be a twisting of nerves, which might be mechanically disadvantageous. For starry flounders, then, the left-eyed variety would be selected against, since in a starry flounder the left optic nerve is uppermost.
The problem with the above explanation is that the Japanese starry flounder population is almost exclusively left-eyed, and natural selection never promotes a purely less advantageous variation. As other explanations
proved equally untenable, biologists concluded that there is no important adaptive difference between left- eyedness and right-eyedness, and that the two characteristics are genetically associated with some
other adaptively significant characteristic. This situation is one commonly encountered by evolutionary biologists, who must often decide whether a characteristic is adaptive or selectively neutral. As for the left-eyed and right-eyed flatfish, their difference, however striking, appears to be an evolutionary red herring.
(456 words）
11. The author would be most likely to agree
with which of the following statements
about left-eyedness and right-eyedness in
the starry flounder?
□A They are adaptive variations by the starry flounder
to environmental differences.
□B They do not seem to give obvious selective
advantages to the starry flounder.
□C They occur in different proportions in different
locations
答案选BC。 主要是我想问一下第三段，那个教授说右边视神经在上面的时候，如果眼睛长在左边，就容易twist。那星状比目鱼的左边视神经在上面，眼睛长在右边就容易twist。既然这样就应该是长在左边的物种被选择啊，但文中为什么说长在左边的物种被淘汰？？？想不通啊想不通..

szmanutdUID: 2727152

For starry flounders, then, the left-eyed variety would be selected against, since in a starry flounder the left optic nerve is uppermost.
说明神经长左边的好

瓶子要学习UID: 2999456

2# szmanutd selected against 应该表示不选择吧 而且如果神经长左边的好，那么下一段的第一句话就不成立了，这个解释就推不出矛盾...而这篇文章就是要说那个解释是错误的

瓶子要学习UID: 2999456

4# fhb881125 诶，好像是哦~~